Unleaded gasoline formulations for piston engines

ABSTRACT

Disclosed herein are fuel formulations comprising C 4 -C 12  aliphatic hydrocarbons and cumidine. In embodiments, the formulations further include a limited amount of aromatic hydrocarbons. In other embodiments, the aliphatic hydrocarbons are alkylates, and in still other embodiment the alkylates are alkanes. These fuel formulations provide unique and advantageous physical properties suitable for piston engines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to lead-free piston engine fuelscomprising hydrocarbon components and cumidine blended together toproduce unique piston-engine motor fuel formulations with a high motoroctane number that offers excellent engine and operational performancefor aviation purposes. These unique fuels have very low freeze points,low environmental toxicity, and a high degree of compatibility withmaterials used in aircraft fuel systems.

2. Description of the Prior Art

Aviation fuels are a product of blending many possible hydrocarboncomponents to very specific formulations to create a combustible fuelthat is tailored for an aviation specific use. For example, turbineengines used on most commercial jets worldwide utilize jet fuelsspecifically designed for their combustion characteristics usinghydrocarbons with longer-chain molecules with carbons typically rangingbetween C₈ to C₁₆. These fuels typically have a high flash point whichmakes them safe for handling in a wide range of commercial uses. Pistonengines used in general aviation require fuels made from lighterhydrocarbons (typically ranging from C₄ to C₁₀ carbon molecules) similarto gasolines used in automobiles, but with much higher octanerequirements and somewhat lower vapor pressure requirements.

For many decades the combustion characteristics of avgas used by pistonengine aircraft has required tetraethyl-lead as a key component to thefuel to achieve the highest levels of motor octane number—therebyhelping to reduce the likelihood of engine knocking. In recent years,the combination of public health hazards and environmental regulationshas triggered an effort across the global aviation industry to removeall lead compounds from avgas.

The alternatives for blending and producing a lead-free aviationgasoline which meets the performance requirements for all varieties ofpiston engine aircraft are complex even for those schooled in the art ofaviation gasolines. Aviation fuels used in piston engine aircraft mustmeet all minimum performance criteria as defined by various fuelspecifications managed by ASTM International and overseen by across-industry forum of experts. The fuel must also meet minimum fueloperating requirements as defined by Federal Aviation Administration(FAA) and other federal, state and local regulators. Specifically theavgas must meet the minimum motor octane number, the appropriate rangefor vapor pressure and all related matters impacting combustion, engineknock suppression, volatility, composition, fluidity, anti-corrosion,oxidation stability, environmental toxicology and materialcompatibility.

To enhance the motor octane rating of avgas for piston aircraft fuelshave included high concentrations of aromatic hydrocarbons (particularlymethylbenzene, dimethylbenzene or 1,3,5-trimethylbenzene), or have beenblended with various aromatic amines (particularly aniline ormeta-toluidine), oxygenates (e.g. MTBE, ETBE and Ethanol) and/or certainmetals (particularly tetraethyl lead). This invention focuses on the useof base compounds using specific C₄ to C₁₀ aliphatic hydrocarbons,preferably blended in the absence of aromatic hydrocarbons (e.g.toluene, xylene trimethylbenzenes), but with the addition of specificaromatic amines to achieve lead-free fuels that meet all the ASTMspecifications for aviation gasoline, including freeze point, whileachieving high motor octane numbers. The fuel is shown to be safe, lowin toxicity and compatible with materials used in aircraft fuel systemsand the related supply chain.

U.S. Pat. No. 5,470,358 describes a wide variety of aromatic amines (atotal of 36 described in research) used in various concentrations toincrease the motor octane number of unleaded aviation gasoline. The basecompound however is an unleaded aviation fuel blend with a minimum 90-93MON which contains up to 20% toluene to which are added aromatic aminesin specified concentrations with claims ranging from 4 to 20% of theamine that results in a high octane avgas. This application fails tolook at the toxicity impact of the fuels with these wide rangingaromatic amines, or the impact these compounds have on freeze point ofthe fuel using ASTM D2386, or the impact of the fuels on enginedeposits, or the exaggerated impact these fuels have on aircraftmaterial compatibility when combined in the presence of toluene, anaromatic hydrocarbon. These factors taken together make this inventionimpractical and commercially undesirable for aviation use.

U.S. patent application Ser. No. 12/093,250 describes various lead-freeaviation fuels with a minimum 100 MON based upon a blend combination ofbase alkylate with aromatic hydrocarbons found in the up to 30%reformate (shown as 35-70% toluene, plus xylenes and C₉'s), plus any ofa broad group of aromatic amines. This application fails to look at thetoxicity impact of amines on the fuels, or the impact these wide rangingamine compounds have on freeze point of the fuel using ASTM D2386, orthe exaggerated impact these fuels have on aircraft materialcompatibility when combining amines with aromatic hydrocarbons. Thesefactors make this invention impractical and commercially undesirable foraviation use.

U.S. Pat. No. 8,628,594 B1 describes an unleaded fuel blended from abase aviation gasoline with a minimum 96 MON which contains variouscombinations of xylenes (notably meta-xylene) and1,3,5-trimethylbenzenes of up to 40% or more, to which is added up to 6%of an aromatic amine (notably m-toluidine). This patent fails to addressthe fact that a) m-toluidine is a highly toxic compound not oftenhandled in the US fuel supply chain, b) the use of m-toluidine causesthe freeze point of the fuel (using ASTM method D2386) to rise above the−58° C. required limit specified by ASTM D910 for aircraft fuel used ataltitude, c) aromatic amines of any amount used in combination witharomatic hydrocarbons will exaggeratedly promote the destructivebehavior on material used in aircraft fuel systems (e.g. gaskets, hoses,o-rings, etc), and d) the results of fuel tests in this research failedto utilize appropriate ASTM test methods to assure a minimum MON of 102as reported by FAA requirements for any unleaded “drop-in” fuel toreplace 100LL avgas. Furthermore, this invention uses non-standardterminology and methods like “FSEEMON” which is arbitrary and outside ofacceptable ASTM and FAA industry norms for comparison of unleaded fueltest results. These factors make this invention impractical andcommercially undesirable for aviation use.

U.S. Pat. No. 9,035,114 B1 describes an unleaded fuel with minimum 99.6MON based upon the use of aniline in combination with 20% to 35% tolueneplus some combinations of branched alkyl acetate compounds. This patentfails to address the fact that 1) aniline is a highly toxic compound nothandled in the US fuel supply chain, b) the use of aniline causes thefreeze point of the fuel (using ASTM method D2386) to rise well abovethe −58° C. required limit by ASTM D910 for aircraft fuel used ataltitude, and c) aromatic amines of any amount used in combination witharomatic hydrocarbons will exaggeratedly promote the destructivebehavior on material used in aircraft fuel systems (e.g. gaskets, hoses,o-rings, etc). These factors make this invention impractical andcommercially undesirable for aviation use.

Many other attempts have been made at devising a lead-free high-octaneaviation gasoline starting from a hydrocarbon-based aviation fuel, someby combining lower boiling alkylates and aromatics up to 80% to increasethe octane, as well as 5-15% of additional C₄-C₅ compounds to reduce thevapor pressure to aviation gasoline standards. See, for example, U.S.Pat. Nos. 8,741,126; 7,416,568; 8,324,437; 8,049,048; and 8,686,202.

SUMMARY OF THE INVENTION

In accordance with the present invention there are provided novel fuelformulations of certain aliphatic hydrocarbons blended with cumidine. Inanother aspect, the present invention provides for an improved fuelcomprising isobutane, isopentane, isooctane, cumidine(4-isopropylaniline), and optionally other aliphatic compounds, andoptionally other aromatics. These formulations provide an unexpectedlyhigh octane, unleaded fuel suitable for off-road motor fuel and aviationgasoline and a wide variety of related products.

In one aspect, the fuels of the present invention are substantially freeof benzene-based aromatic hydrocarbons having 6 or more carbon atoms.Despite years of industry testing using aromatic amines for fuels, theindustry has not seen the fact that substantially excludingbenzene-based aromatic hydrocarbons and selectively including onlycumidine as a fuel component provides a unique and practical solution tothe search for a viable, high-octane, lead-free aviation gasoline.

It is an object of the present invention to provide improved fuelshaving numerous advantageous properties, and which are useful asaviation fuel for many types of aircraft engines includinghigh-performance engines and also legacy aircraft.

Another object of the invention is to provide formulations of thepresent invention having suitable boiling point characteristics, therebyfavorably impacting fuel stability, cold starting features, etc.

A further object of the present invention is to provide fuelformulations having surprisingly high motor octane numbers (MON) andresearch octane numbers (RON).

Another object of the present invention is to provide an improved fuelthat contains a minimal amount of lead compounds to achieve its optimaldetonation suppression characteristics. For example, certainformulations of the present invention do not include the use of anytetraethyl lead, or any ethylene dibromide to scavenge for lead in theaircraft fuel system.

In still another aspect, the present invention provides for an improvedfuel that meets or exceeds most or all of the requirements of ASTM D910,ASTM D7719, D7592 and/or ASTM D7547.

Additional embodiments of the invention, as well as features andadvantages thereof, will be apparent from the descriptions herein.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to certain embodiments andspecific language will be used to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications, and such further applications of the principles of theinvention as described herein, being contemplated as would normallyoccur to one skilled in the art to which the invention relates.

Motor fuels are used in a variety of systems. In the broadest sense, amotor fuel is one which is used in piston or turbine engines. Thepresent invention is directed to fuels for piston engines useful inoff-road ground vehicles and/or aircraft. Typically, ground vehicles canuse relatively lower octane fuels, while aircraft require higher octanefuels. A basic determinant as to the choice of fuels is the octanerating of the fuel compared to the compression of the engine. Forexample, higher compression engines generally require higher octanefuels. This invention provides fuels suitable generally for pistonengines. Certain embodiments are particularly applicable for use inaircraft engines.

The present invention provides unleaded, piston engine fuels preferablycomprising a mixture of select aliphatic hydrocarbons blended withcumidine. The aliphatic hydrocarbons may include alkanes, alkenes,alkynes, cycloalkanes and alkadienes. In preferred embodiments, thealiphatic hydrocarbons comprise lower boiling C₄ to C₁₀ alkanes, alkenesand cycloalkanes, but largely excluding arenes found in gasoline. Thecumidine isomer used in the fuels is preferably 4-isopropylaniline. Theresulting fuel formulations are characterized by an array of desirableproperties making them suitable for piston engines.

In certain aspects, the fuels comprise an alkylate product consisting ofa variety of hydrocarbons. In refining, the alkylation processtransforms low molecular-weight alkenes and iso-paraffin molecules intoa product referred to as an “alkylate”, which includes a mixture ofhigh-octane, isoparaffins. This “alkylate” product may contain manyhydrocarbon compounds typically in the C4 to C12 range.

“Aviation alkylate” is a premium gasoline blending stock havingexceptional anti-knock properties and a final boiling point appropriatefor aviation use. The octane number of an aviation alkylate dependsmainly upon the kind of alkenes used and upon refinery operatingconditions. For example, isooctane typically results from combiningisobutylene with isobutane and has an octane rating of 100 bydefinition. There are other products in the alkylate, so the octanerating will vary accordingly.

As used herein, the term “alkylate” refers to the alkylate productavailable from a refinery, and also generally to any mixture includingC4 to C12 non-aromatic hydrocarbons. These alkylates are useful in theformulations to address the problem of cold starts. Whether from thealkylate product of the refineries, or in more purified form, theinclusion of high volatility/low boiling point components contributes toachieving a desired Reid Vapor Pressure (RVP) range, while also allowingthe engines to start in cold temperature situations (cold weather orhigh altitude).

In one aspect, the alkylate component comprises alkanes. In particular,it has been found that the C4-C10 alkanes, and more preferably branchedalkanes, provide especially desirable properties for the inventive fuelformulations. Isobutane, isopentane and/ isooctane are particularlypreferred in order to achieve a balance of desirable fuel properties. Inone embodiment, the formulations comprise about 7 wt % to about 13 wt %C4-05 branched chain alkanes by weight.

The formulations also include up to 20% cumidine, more preferably up toabout 15 wt % cumidine. Cumidine refers to three isomeric liquid bases(C₃H₇C₆H₄NH₂) derived from cumene. It has been discovered that cumidinehas unique properties for an aromatic amine related to high octaneaviation gasoline. In the present invention, the isomer4-isopropylaniline is preferably used.

The following Table 1 highlights a number of aromatic amines which mightappear to be desirable as octane enhancing components to aviationgasoline (compared to baseline avgas). However, none of these amines aredesirable for aviation gasoline, except cumidine. Only two components(4-isopropylaniline and N-ethylphenylamine) have freeze points that arelow enough to be desirable for aviation gasoline when flying in highaltitude. Also five of the amines have water solubility problems makingthem unacceptable for consideration as aviation gasoline due to concernfor attracting water which can result in inflight freezing. Thirteen ofthe compounds are fluoro- or chloro-compounds that are not acceptablecandidates for any commercial fuel use. Two compounds have final boilingpoints that exceed guidelines for all aviation gasolines. The onlyviable aromatic amine component that meets the critical technicalcriteria for aviation gasoline is 4-isopropylaniline (cumidine).

TABLE 1 Freeze Octane Octane Boiling Aromatic Amines CAS Point @ 6%* @9%* Other Point phenylamine 62-53-3 −6° C. 5.70 8.70 184° C.N-methylphenylamine 100-61-8 −57° C.  2.10 2.60 196° C.2-methylphenylamine 95-53-4 −16° C.  1.70 2.10 200° C.N-ethylphenylamine 108-69-5 −63.5° C.   −2.50 — Insoluable 205° C. >6%3,5- 108-69-0  9° C. 5.70 8.70 220° C. dimethylphenylamine 3,4- 95-64-750° C. 5.60 — Solid 226° C. dimethylphenylamine @RT 3-methylphenylamine108-44-1 −31° C.  5.40 7.40 203° C. 3-ethylphenylamine 587-02-0 −8° C.3.80 6.60 212° C. 4-methylphenylamine 106-49-0 45° C. 6.10 — solid @ RT200° C. 4-ethylphenylamine 589-16-2 −5° C. 3.80 6.30 217° C.4-isopropylaniline 99-88-7 −63° C.  4.40 7.20 227° C. (Cumidine)4-t-butylphenylamine 769-92-6 15° C. 4.20 6.60 235° C.2-fluorophenylamine 348-54-9 −29° C.  4.20 7.30 182° C.3-fluorophenylamine 372-19-0 −2° C. 4.20 6.70 186° C.4-fluorophenylamine 371-40-4 −2° C. 4.20 7.50 187° C.2-chlorophenylamine 95-51-2 −3° C. 4.50 5.60 209° C. 3-chlorophenylamine108-42-9 −10° C.  3.60 — Insoluable 230° C. >6% 2-fluoro-4- 452-80-2  3°C. 4.00 5.10 206° C. metylphenylamine 2-fluoro-5- 452-84-6 3.60 5.10202° C. metylphenylamine 3-fluoro-2- 443-86-7  7° C. 1.30 2.70 188° C.metylphenylamine 4-fluoro-2- 452-71-1 14° C. 1.60 2.10 207° C.metylphenylamine 5-fluoro-2- 367-29-3 37° C. 1.60 2.10 235° C.metylphenylamine 2,3,4,5- 5580-80-3 28° C. 1.70 2.50 solid @ RT 213° C.tetrafluorophenylamine N-methyl-4- 459-59-6 2.50 2.60 181°fluorophenylamine 2-fluoro-4-methyl- 452-80-2  3° C. 4.00 5.10 222° C.phenylamine

Formulations

The fuel formulations of the present invention generally comprise about80 to about 99 wt % C₄-C₁₂ aliphatic hydrocarbons, about 1 to about 20wt % cumidine, and less than about 5 wt % C₆-C₁₂ aromatic hydrocarbons,and the formulations are substantially free of lead-containingconstituents. In preferred embodiments, the formulations aresubstantially free of C₆-C₁₂ aromatic hydrocarbons. In a further aspect,the fuel formulations consist essentially of about 80 to about 99 wt %C₄-C₁₂ aliphatic hydrocarbons and about 1 to about 20 wt % cumidine.More particularly, the formulations preferably consist essentially ofabout 85 to about 95 wt % C₄-C₁₂ aliphatic hydrocarbons and about 5 toabout 15 wt % cumidine. The fuel formulations have a MON of at leastabout 100 and an RVP of 38 to 49 kPa at 37.8° C.

In another aspect, the piston engine fuel formulations of the presentinvention comprise about 80 to about 99 wt % C₄-C₁₀ alkylates, about 1to about 20 wt % cumidine, and less than about 5 wt % C₆-C₁₂ aromatichydrocarbons. More preferably, the fuel formulations comprise about 80to about 99 wt % C₄-C₁₀ alkanes, about 1 to about 20 wt % cumidine, andless than about 5 wt % C₆-C₁₂ aromatic hydrocarbons.

In certain combinations, the fuel formulations comprise about 80 toabout 99 wt %, more preferably about 85 to about 95 wt %, isobutane,isopentane and/or isooctane and about 1 to about 20 wt %, morepreferably about 5 to about 15 wt %, cumidine. The cumidine preferablyis 4-isopropylaniline. These formulations also are preferablysubstantially free of C₆-C₁₂ aromatic hydrocarbons. In a further aspect,the fuel formulations consisting essentially of about 85 to about 95 wt% isobutane, isopentane and/or isooctane and about 5 to about 15 wt %4-isopropylaniline. Said formulations also preferably comprise a MON ofat least about 100 and an RVP of 38 to 49 kPa at 37.8° C.

Various specific fuel formulations have also been identified. One suchformulation includes, and more preferably consists essentially of, about2 wt % isobutane, about 10 wt % isopentane, about 73 wt % isooctane, andabout 15 wt % 4-isopropylaniline, and has a MON of about 103.5. In yetanother embodiment, the formulations comprise a blend of 30% low-boilingalkylate, 45% isooctane, 10% isopentane, 2% isobutane and 12.5% cumidineby weight have a MON of 102.5. These embodiments preferably aresubstantially free of aromatic hydrocarbons.

In another aspect of the invention, there is provided an unleaded,piston engine fuel formulation comprising a blend of isooctane,4-isopropylaniline, isobutane, isopentane and at least one othercomponent selected from the group consisting of alkylates or alkanes andhaving a MON of at least 94 and an RVP of 38 to 49 kPa at 37.8° C. In arelated aspect, this formulation consists essentially of isooctane,4-isopropylaniline, isobutane, isopentane and up to 5% by weight of atleast one additive selected from the group consisting of octaneboosters, antioxidants, co-solvents, toluene, xylene, electricalconductivity additives, corrosion inhibitors, metal deactivators, dyes,and any combinations and mixtures thereof. Specifically, the latterembodiments may comprise alkylates or alkanes, or a combination ofalkylates and alkanes but excluding C7-C9 aromatic hydrocarbons.

In another aspect, the fuel formulations of the present inventioncontain the following ranges of components:

(Iso)butane: 0-2 wt %

Isopentane: 5-15 wt %

Isooctane/Alkylate: 60-75 wt %

Cumidine: 5-15 wt %

Other Aromatics: 0-5 wt %

In yet another aspect, the fuel formulations of the present inventioncontain the following ranges of components:

(Iso)butane: 0-2 wt %

Isopentane: 5-15 wt %

Isooctane/Alkylate: 60-75 wt %

Cumidine: 5-15 wt %

Other Aromatics: 0-5 wt %

Plus up to 250 ppm of ferrocene additives

These formulations are hereafter referred to as formula UL102AA, and itsproperties are compared with relevant ASTM standards for certain pistonengine fuels.

In further embodiments, the fuel formulation comprises, or consistsessentially of, about 2 wt % isobutane, about 10 wt % isopentane, about73 wt % isooctane, and about 15 wt % 4-isopropylaniline, and having aMON of about 103.5. In other embodiments, the fuel formulationcomprises, or consists essentially of, about 2 wt % isobutane, about 10wt % isopentane, about 76 wt % isooctane, and about 12 wt %4-isopropylaniline, and having a MON of about 102.

One basic goal of the inventive fuels lies in balancing the synergisticeffects of a number of components and compounds to achieve, as closelyas possible, the performance properties of the historic ASTM D910 fueland/or other ASTM standards which may be applicable.

ASTM specification D910 is entitled “Standard Specification for AviationGasolines” and describes several characteristics that an aviationgasoline may meet, and is hereby incorporated by reference in itsentirety. ASTM D910 also makes reference to documents, for example butnot limited to other ASTM specifications, and these references are alsohereby incorporated by reference.

The ASTM D910 specification describes many requirements, including thefollowing. The distillation curve has specified vol % levels of theevaporation test (10%, 40%, 50%, 90% and final boiling point) withminimum and/or maximum fuel temperature requirements such as 75° C.,105° C., 135° C., and 170° C. The distillation curve has a minimumrecovery volume of 97%, a residue volume maximum of 1.5%, and a maximumloss of 1.5%. The fuel composition has a freezing point below −58° C.and a net heat of combustion of at least 43.5 MJ/kg. The fuelcomposition has an appropriate density, a sulfur content less than0.05%, and an oxidation stability of about 6 mg/100 mL. The fuelcomposition exhibits corrosion of a copper strip for 2 hours at 100° C.less than the value indicated in ASTM D910. The fuel compositionexhibits a water reaction of less than +/−2 volume changes in mL/100 mL,and an electrical conductivity of less than 450 pS/m. The fuelcomposition exhibits a Reid Vapor Pressure (RVP) between 38 to 49 kPa at38° C. The Motor Octane Number is a minimum of 99.6. The maximum contentof tetraethyl lead (TEL) is 0.53 mL/L.

The following Table 2 compares UL102AA to ASTM D910 (Grade 100LL) as toall the fuel performance properties and related to temperature ASTMD6227 (Grade UL87).

UL102AA has a minimum MON that is 2.9 octane numbers higher than thatspecified for 100LL, as required to achieve or exceed equivalent knockresistance compared to a leaded fuel.

UL102AA has a net heat of combustion minimum of 43.5 MJ/kg, which isequivalent to the specified limit for 100LL. The fuel has a density of700-720 kg/m³, also equivalent to 100LL, which together results in noimpact on aircraft weight or range.

The distillation curve for UL102AA varies from the curve for 100LL onlyat the final boiling point. While the maximum final boiling pointallowable for UL102AA is currently higher than ASTM D910, it should benoted that other widely used and certified aviation gasolinespecifications such as GOST 1012-72 and ASTM D6227 also support higherfinal boiling points than historically permitted by ASTM D910, as doesthe production specification ASTM D7719.

UL102AA is an unleaded fuel that allows for up to 0.013 gPb/L in case ofaccidental contamination between the refinery and the FBO, whereas 100LLis a leaded fuel that can contain up to 0.56 gPb/L. High-Octane UL102AA,being an unleaded fuel, will have zero lead precipitate.

UL102AA contains less than 20% (m/m) aromatics largely from a uniquenitrogen-based aromatic amine; in fact the preferred formulationcontains less than 15% cumidine. This is well within the norm ofindustry standards for aviation gasolines.

The freezing point for UL102AA, using the standard method ASTM D2386, is−58° C. maximum, which is identical to ASTM D910.

The electrical conductivity for UL102AA is 450 pS/m maximum, which isidentical to ASTM D910.

TABLE 2 ASTM Test High-Octane Method ASTM Requirements ASTM D910Unleaded 102AA Grade 100LL, Avgas 102AA COMBUSTION D4809 Net Heat ofCombustion, MJ/kg 43.5, min 43.5, min Octane Rating D2700 Knock value,lean mixture Motor Octane Number 99.6, min 102.5, min COMPOSITION D2622Sulfur, mass % 0.05, max 0.005, max D5059 Tetraethyl lead, mL g Pb/L0.56, max 0.013, max VOLATILITY D5191 Vapor pressure, 38° C., kPa 38-4938-49 D1298 Density at 15° C., kg/m³ Report Report D86 DistillationInitial Boiling Point, ° C. Report Report Fuel Evaporated 10, volume %at ° C. 75, max 75, max 40, volume % at ° C. 75, min 75, min 50, volume% at ° C. 105, max 105, max 90, volume % at ° C. 135, max 135, max Finalboiling point, ° C. 170, mix 225, max Sum of 10% + 50% evaporated 135,min 135, max temperatures, ° C. Recovery, volume % 97, min 97, minResidue, volume % 1.5, max 1.5, max Loss, volume % 1.5, max 1.5, maxFLUIDITY D2386 Freezing Point, ° C. −58, max −58, max CORROSION D130Corrosion, copper strip, 2 h No. 1, max No. 1, max @100° C. CONTAMINANTSD873 Oxidation, stability (5 h aging) Potential gum, mg/100 mL 6, max 6,max D1094 Water reaction Volume change, mL ±2, max ±2, max OTHER D2624Electrical conductivity, 19.9° C., 450, max 450, max pS/m

The present invention has been developed to provide an unleaded aviationgrade fuel for engine/aircraft types capable of operating as a drop-inreplacement for ASTM D910 Grade 100LL leaded aviation gasoline (Avgas).The unleaded fuels of this invention meet the performancecharacteristics of the ASTM D910 standard, except for lead content andfinal boiling point. However, this property as outlined in D910 is not acritical operating factor, as other unleaded aviation gasolines havealready been approved with such higher final boiling point limits. Thefuel additionally offers benefits of 100% elimination of the use oftetraethyl lead while meeting the anti-knock characteristics of thehigh-performance piston engine aircraft. See Table 2.

ASTM specification D7719 describes a fuel specification for high octaneaviation fuel, and is hereby incorporated by reference in its entirety.ASTM D7719 also makes reference to documents, for example but notlimited to other ASTM specifications, and these references are herebyincorporated by reference in their entirety.

ASTM specification D7592 describes a fuel specification for unleadedaviation fuel. ASTM D7592 is hereby incorporated by reference in itsentirety. ASTM D7592 also makes reference to documents, for example butnot limited to other ASTM specifications and these references are herebyincorporated by reference in their entirety.

ASTM specification D7547 describes a fuel specification for unleadedaviation fuel. ASTM D7547 is hereby incorporated by reference in itsentirety. ASTM D7547 also makes reference to documents, for example butnot limited to other ASTM specifications and these references are herebyincorporated by reference in their entirety.

MON and Anti-Knock

Motor fuel must meet the power demands for the selected engines. Themotor octane number, or MON, is a standard measure of the performance ofa fuel. A gasoline-fueled reciprocating engine requires fuel ofsufficient octane rating to prevent uncontrolled combustion known asengine knocking (“knock” or “ping”). The higher the MON, the morecompression the fuel can withstand before detonating. In broad terms,fuels with a higher motor octane rating are most useful inhigh-compression engines that generally have higher performance. The MONis a measure of how the fuel behaves when under load (stress). ASTM testmethod 2700 describes MON testing using a test engine with a preheatedfuel mixture, 900 rpm engine speed, and variable ignition timing tostress the fuel's knock resistance. The MON of an aviation gasoline fuelcan be used as a guide to the amount of knock-limiting power that may beobtained in a full-scale engine under take-off, climb and cruiseconditions.

A particular aspect of the present invention is to provide formulationswhich are useful as piston engine fuels, and are particularly suited foruse as aviation gasoline. Aviation gas, or avgas, has a number ofspecial requirements as compared to ground vehicle gasoline. Aviationgasoline is an aviation fuel used in spark-ignited (reciprocating)piston engines to propel aircraft. Avgas is distinguished from mogas(motor gasoline), which is the everyday gasoline used in motor vehiclesand some light aircraft. It has been determined that 4-isopropylanilinehas a uniquely positive impact on motor octane number—and thereby engineknock resistance.

Various MON ratings are considered to be base requirements for aircraftuse, depending on the type of engine and other factors. The presentinvention provides aviation fuels which have a MON of at least 100,preferably 102 or greater. A second consideration can be the researchoctane number (RON), which is determined similarly to MON but underlower engine load or reduced stress (i.e., lower RPM, lower temperature,fixed ignition). The fuels of the present invention meet MON and RONrequirements.

RVP

The vapor pressure of a fuel is another important factor for avgas.Aircraft engines operate in wide ranges of temperatures and atmosphericpressures (e.g., altitudes), and the fuels must start and providesufficient combustion characteristics throughout those ranges. Lowervapor pressure levels are desirable in avoiding vapor lock during summerheat and/or high altitude flying, and higher levels of vaporization aredesirable for winter starting and operation. Fuel cannot be pumped whenthere is vapor in the fuel line (summer) and winter starting (“coldstart’) will be more difficult when liquid gasoline in the combustionchambers has not vaporized. Vapor pressure is critically important foraviation gasolines, affecting starting, warm-up, and tendency to vaporlock with high operating temperatures or high altitudes.

The ability of an aviation gas to satisfy the foregoing requirements maybe assessed based on the Reid Vapor Pressure (RVP). The Reid VaporPressure is the absolute vapor pressure exerted by a liquid at 37.8° C.(100° F.) as determined by the test method ASTM-D5191. The RVP differsfrom the true vapor pressure due at least in part to the presence ofwater vapor and air in the confined space. A typical requirement foravgas is that it has an RVP of 38-49 kilopascals (kPa) at 37.8° C., asdetermined in accordance with applicable ASTM standards. Theformulations of the present invention meet RVP requirements for aviationgas.

Combustion Performance

The fuels of the present invention have suitable combustion performance.The present invention provides fuels having a net heat of combustion bymass of 43.5 MJ/kg, which is equivalent to 100LL. The density of thefuel is equivalent to 100LL; therefore, the weight and range of theaircraft will be identical to that of 100LL.

Cold Start

Preliminary testing of the fuels of this invention conducted in anengine test cell indicated that the fuel achieved cold start at −20° C.,and the preliminary engine performance results were “positive”.

Fluidity

Fluidity is a critical operating parameter for flight safety. Avgas mustnot freeze at cold temperatures typical of high altitude operations. Allindustry standards for avgas call for fuel to remain in a liquid statedown to −58° C. according to the ASTM freeze point test method D2386.Many aromatic amines have freeze points much higher than −58° C. (e.g.aniline freezes is −6° C.) which tends to add complexity for thesecompounds to be useful components for blending into avgas formulations.Cumidine has a freeze point of −63° C. making it a unique and idealblending component for avgas (e.g., m-toluidine is −33° C. and anilineis −6° C.). This allows the blended fuels of the present invention toadhere to rigorous ASTM fuel requirements with a freeze point at −58° C.

The fluidity of fuel is consistent with 100LL, with a freezing pointmaximum of −58° C. None of the components of the inventive fuels(including cumidine) have a freeze point above −60° C., which allows thefuel to meet the rigorous requirement necessary to ensure flow in aliquid state during high-altitude/low temperature operations.

Volatility

Volatility of the fuels of this invention is another critical operatingparameter for reliability and flight safety. The fuels meet thetraditional aviation gasoline standard of 38-49 kPa, particularly withthe presence of isopentane and not more than 2% isobutane. Tests revealthat fuels with isobutane concentrations higher than 3% will exceed themaximum vapor pressure limit and experience loss >1.5%. Fuels that aretoo volatile can experience vapor lock causing problems in normaloperations, or causing the engine not to start on the ground, or notrestarting in an emergency situation at altitude.

Insolubility

Avgas must also be highly insoluble in water. Water dissolved inaviation fuels can cause serious problems, particularly at altitude. Asthe temperature lowers, the dissolved water becomes free water. Thisthen poses a problem if ice crystals form, clogging filters and othersmall orifices, which can result in engine failure. The present fuelsare insoluble in water.

Material Compatibility

Avgas must function in the engine and fuel system without reactingunfavorably to the materials in these systems. Fuels typically result inseal swell, a favorable trait for reducing leaks. However, aromaticamines can have destructive tendencies. This invention discovered theunique requirement to reduce or eliminate the use of aromatichydrocarbons with aromatic amines—thereby eliminating the tendency topromote destructive behaviors to such materials.

Previous tests using base alkylates with aromatics in combination witharomatic amines caused an exaggerated and destructive reaction tomaterial compatibility results impacting fuel system components and hadgenerally high toxicity creating environmental challenges in the fuelsupply chain.

Many others have tried unsuccessfully to produce fuels with aromaticamines. However, components such as aniline and m-toluidine tend to behighly corrosive and/or destructive to fuel systems due in some part totheir molecular size and polarity. They overly exaggerate seal swell andreduce tensile strength in buna/vinyl rubber and neoprene fuel systemswhen mixed with toluene.

Oxidation Stability

The fuels of this invention meet the strict oxidation stabilityrequirements of the aviation fuel specification with a very lowpotential for insoluble gums (maximum 2 mg/100 ml, using ASTM methodD873) but without the risk of lead precipitate, as it is an unleadedfuel.

Corrosion

The corrosion testing done shows that the fuels of this invention meetthe strict D910 standard for accelerated soak testing of a copper strip.Corrosion inhibitors that conform to the latest issue of D910, D7592 orD7547 may be added to the fuel in amounts not exceeding the maximumallowable concentrations listed.

Pre-Combustion and Post Combustion Properties

It is not uncommon in aviation that fuel can be inadvertently spilled inclose proximity to persons when maintaining or refueling the aircraft.The fuel is a flammable hydrocarbon-based liquid, with the addition ofcumidine as an octane enhancer. It evaporates quickly. As with all fuelscontaining aromatic amines, as the fuel evaporates, the amine evaporatesmore slowly. Care should be taken to avoid contacting this concentratedamine with skin. In consideration of the environmental toxicology of thefuel, cumidine was chosen for the fuel, rather than aniline, m-toluidineor other aromatic amines, due in part to its overall lower toxicity.

The fuels, according to test results, has slightly less smoke than 100LL(using ASTM D1322—smoke point test), albeit with less-toxic emissionsdue to the absence of lead and its scavengers (e.g. diethylene bromide).

The base fuel has pre- and post-combustion occupational exposure limitssimilar to those of automotive gasoline, which typically range from 25ppm-300 ppm [TWA: 8 hours OSHA]. The TWA for cumidine is estimated to be2 ppm. Cumidine is not classified as a mutagen or carcinogen. It is 88%biodegradable within 10 days, has an LD₅₀ (rat, oral) of 757 mg/kg, isreadily eliminated from water, and is not expected to bioaccumulate. See4-Isopropylaniline; MSDS No. 818472 [Online]; EMD Millipore: Billerica,Mass., Aug. 22, 2013, http://www.emdmillipore.com.

Toxicity

Aromatic amines tend to be highly toxic (e.g. aniline is defined byMerck Index as “poisonous” and by OSHA as an irritant). The fuelcompositions of the present invention contain cumidine, which is a lowtoxicity aromatic. Below in Table 3 is a brief recap of the overalltoxicity of many candidate fuel components.

TABLE 3 Component LD₅₀ (rat, oral) OSHA Hazards Mesitylene 5,000 mg/kgIrritant ETBE 5,000 mg/kg Irritant Toluene 5,000 mg/kg Irritant,Teratogen, Reproductive hazard Benzene 2,990 mg/kg Carcinogen, Mutagen,Irritant Cumidine 757 mg/kg Irritant. Causes respiratory tractirritation. Causes eye and skin irritation. Can form methemoglobin, maycause cyanosis. May cause central nervous system depression. m-Toluidine450 mg/kg Toxic. Causes cyanosis. Harmful or fatal if inhaled,swallowed, or absorbed through skin. May be irritating to skin, eyes andmucous membranes. Target organs: Bladder; kidneys; blood; liver. Aniline250 mg/kg Carcinogen, toxic if swallowed, toxic in contact with skin,causes skin irritation, causes serious eye damage, fatal if inhaled,suspected of causing genetic defects. Dibromoethane 55 mg/kg Carcinogen,toxic by inhalation, toxic by skin absorption. TEL 14 mg/kg Carcinogen,toxic by inhalation, highly toxic by ingestion, highly toxic by skinabsorption, teratogen. Source: MSDS data from Sigma-Aldrich, et al.

Table 3 highlights the relative acute toxicity based on public datausing LD₅₀ as an internationally accepted baseline. In addition, chroniceffects from long term exposure and other effects like carcinogenicity,mutagenicity, and teratogenicity have to be considered for the objectiveevaluation of the fuel.

Another key factor is the relative concentration of potentially toxiccomponents in a particular fuel formulation, e.g., certain aromaticamines may require 60 to 250 times the concentration level in ahigh-octane unleaded aviation fuel vs. TEL found in 100LL. See Albuzat,T., Understanding the Merits of 1,3,5-Trimethylbenzene. CRC AviationMeetings, Apr. 28, 2014, p. 6.

In summary, the present invention provides fuel formulations which arecapable of meeting all of these strict requirements. They meet the MONstandards, have suitable RVP, have low freeze points, acceptablematerial compatibility and are not soluble in water. The fuels of thisinvention further demonstrated the following results compared to 100LL.Cold Start: Both fuels started below −20° C. Exhaust Gas Temp (EGT): thefuel ran on average 16-32° C. hotter than 100LL. Cylinder Head Temp(CHT): the fuel ran on average 8-15° C. hotter than 100LL. Fuelconsumption: ran equivalently for both fuels. This test experiencedoccasional misfires on 100LL, which reduced the EGT and CHT of 100LL,which explains the minor discrepancies.

In a preferred embodiment, the formulations of the present inventionmeet the specifications set forth in ASTM D7719 for a high aromatic,unleaded hydrocarbon based aviation fuel.

These fuels may optionally include other components or additives,particularly to modify or enhance characteristics such as octane rating,vapor pressure, viscosity, anti-icing, anti-static, oxidation stability,anti-corrosion, boiling point, engine cold start, exhaust smoke andengine deposits.

Oxidation Inhibitors

Oxidation inhibitors may be added to the fuel separately, or incombination, in total concentration not to exceed 12 mg of inhibitor(not including weight of solvent) per liter of fuel, such as:

(1) 2,6-ditertiary butyl-4-methylphenol.

(2) 2,4-dimethyl-6-tertiary butylphenol.

(3) 2,6-ditertiary butylphenol.

(4) 75% minimum 2,6-ditertiary-butylphenol plus 25% maximum mixedtertiary and tritertiary butylphenols.

(5) 75% minimum di- and tri-isopropyl phenols plus 25% maximum di- andtri-tertiary butylphenols.

(6) 72% minimum 4-dimethyl-6-tertiary butylphenol plus 28% maximummonomethyl and dimethyl tertiary butylphenols.

(7) N,N′-di-isopropyl-para-phenylenediamine.

(8) N,N′-di-secondary-butyl-para-phenylenediamine.

Icing Inhibitors

Fuel system icing inhibitors may also be used, e.g.:

(1) isopropyl Alcohol (IPA, propan-2-ol), in accordance with therequirements of Specification D4171 (Type II). Fuel system icinginhibitors may also be used in concentrations recommended by theaircraft manufacturer when required by the aircraft owner/operator.

(2) Di-Ethylene Glycol Monomethyl Ether (Di-EGME), conforming to therequirements of Specification D4171 (Type III). May be used inconcentrations of 0.10 to 0.15 volume % when required by the aircraftowner/operator.

Octane Boosters

A variety of fuel additives have been known and used in the art toincrease octane ratings, and thereby reduce knocking. Typical “octanebooster” gasoline additives include methyl tert-butyl ether (MTBE) andethyl tert-butyl ether (ETBE), both of which are known as oxygenateswhich are compounds that contain oxygen as part of their chemicalstructure. Oxygenates help gasoline burn more completely, reducingtailpipe emissions.

Some embodiments of the present invention may utilize non-leadedcombustion enhancing additives individually or in combination with up to6% by weight, such as esters, ethers, carbonates, C5-C7 cycloalkanes, orthe use of triptane and other known octane boosters.

The inventive fuels may “comprise” the described formulations, in whichother components may be included. However, in a preferred embodiment,the inventive fuels “consist of” the described formulations, in which noother components are present. In addition, the inventive fuels may“consist essentially of” the formulations, in which case other fuelexcipients may be included. As used herein, the term “fuel excipients”refers to materials which afford improved performance when used withfuels, but which do not directly participate in the combustionreactions. Fuel excipients thus may include, for example, antioxidants,etc.

Blends

The formulations are also useful for combining with other fuelcomponents to form blends that are useful as motor fuels, including asaviation gasoline. As used herein, the term “fuel components” refers tomaterials which are themselves combustible and have varying motor octaneratings and are included primarily to provide improved combustioncharacteristics of the blend. In preferred embodiments, such fuelcomponents are present in the blend at less than 5 wt %, and morepreferably less than 1 wt %.

Blending of the formulations described herein can be performed in anysuitable order. The examples and exemplary language provided herein areintended to better illuminate the invention and do not pose a limitationon the scope of the invention unless otherwise claimed. No language inthe specification should be construed as indicating any non-claimedelement as essential to the practice of the invention.

Lead

Most grades of avgas have historically contained tetraethyl lead (TEL),a toxic substance used to prevent engine knocking (detonation). Thisinvention produces an unleaded grade of avgas with fuel properties thatmeet minimum power rating (motor octane number), appropriate combustionanti-knocking (detonation suppression), volatility (vapor pressure), andrelated criteria. The inventive fuels allow a range of piston engineaircraft, including those with high-compression engines, to performeffectively to manufacturer requirements. It is necessary that avgasprovide sufficient power under varying conditions, including take-offand climb as well as at cruise.

Tetraethyl lead, abbreviated TEL, is an organolead compound with theformula (CH₃CH₂)₄Pb. It has been mixed with gasoline since the 1920's asan inexpensive octane booster which allowed engine compression to beraised substantially, which in turn increased vehicle performance andfuel economy. Over the years, certain of these leaded fuel grades havebeen referred to as low lead, or “LL”. One advantage of TEL is the verylow concentration needed. Other anti-knock agents must be used ingreater amounts than TEL, often reducing the energy content of thegasoline. However, TEL has been in the process of being phased out sincethe mid-1970s because of its neurotoxicity and its damaging effect oncatalytic converters. Most grades of avgas have historically containedTEL.

This invention advantageously produces an unleaded grade of avgas whichallows a range of piston engines, including high-compression engines, toperform effectively. Therefore, in a preferred embodiment the inventiveformulations and blends are unleaded, i.e., free of TEL. This is madepossible, at least in part, by the presence of the cumidine, whichprovides sufficiently high MON performance and anti-knockingcharacteristics under stress to offset the absence of TEL in theaviation gasoline. It is an object of the present invention to provideavgas formulations that do not require deleterious octane boosters, andwhich meet or exceed requirements for aviation gasoline.

The fuel formulations may also include a limited amount of aromatichydrocarbons, e.g., toluene, xylene, trimethylbenzenes, etc. Thesecompounds are frequently found in minor amounts in product streamsuseful for the present formulations. Moreover, in preparing fuels it isnot economical to use analytical grade or reagent grade chemicals, oreven technical grade chemicals, as the presence of other fuel-compatiblecomponents is not a concern, provided the resulting fuel formulationmeets ASTM and other applicable standards. Thus, the present inventioncontemplates the presence of such other fuel-compatible components inlimited amounts, e.g., less than 5 wt %, preferably less than 2 wt %,and more preferably less than 1 wt %.

Fuel components typically are not chemically pure, but instead maycontain of other, non-deleterious fuel components. The term“non-deleterious fuel components” refers to components which are presentin a formulation other than as an intended component. Thus, selectedadditives such as mentioned above are not encompassed by this term.Instead, it refers more particularly to the fact that materials used incommercial embodiments of piston engine fuels may include constituents,e.g., hydrocarbons, which are present as contaminants to the componentsof primary interest. For example, an alkylate stream from a refinery maybe primarily composed of desired isobutane, isopentane and/or isooctane,but may contain limited amounts of other hydrocarbons such as aromatichydrocarbons. As used herein, the term “substantially free of” refers tothe fact that the amount of such non-deleterious fuel components is lessthan about 5 wt %, preferably less than 2 wt % and more preferably lessthan 0.5 wt %, of the weight of the overall fuel formulation.

In another aspect, it has been determined that the presence of C6-12aromatic hydrocarbons (e.g., toluene, xylenes or mesitylene) with thecumidine can result in material compatibility results that exaggerateseal swell and demonstrate signs of material erosion. It is thereforeadvantageous in certain embodiments of the present invention that thefuel formulations are substantially free of C6-12 aromatic hydrocarbons.

All component percentages expressed herein refer to percentages byweight of the formulation, unless indicated otherwise. Given thesimilarity of the densities of the components of the present invention,it will be appreciated that the use of volume or weight percents of thecomponents in the ranges indicated provide comparable results.

The uses of the terms “a” and “an” and “the” and similar references inthe context of describing the invention (especially in the context ofthe following claims) are to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. Recitation of ranges of values herein are merely intended toserve as a shorthand method of referring individually to each separatevalue falling within the range, unless otherwise indicated herein, andeach separate value is incorporated into the specification as if it wereindividually recited herein.

While the invention has been illustrated and described in the foregoingdescription, the same is to be considered as illustrative and notrestrictive in character, it being understood that only certainpreferred embodiments have been described and that all changes andmodifications that come within the spirit of the invention are desiredto be protected. In addition, all references cited herein are indicativeof the level of skill in the art and are hereby incorporated byreference in their entirety.

1. A piston engine fuel formulation comprising: about 80 to about 99 wt% C₄-C₁₂ aliphatic hydrocarbons; about 1 to about 20 wt % cumidine; andless than about 5 wt % C₆-C₁₂ aromatic hydrocarbons, said fuelformulation being substantially free of lead-containing constituents. 2.The fuel formulation of claim 1 being substantially free of C₆-C₁₂aromatic hydrocarbons.
 3. The fuel formulation of claim 1 consistingessentially of: about 80 to about 99 wt % C₄-C₁₂ aliphatic hydrocarbons;and about 1 to about 20 wt % cumidine.
 4. The fuel formulation of claim1 consisting essentially of: about 85 to about 95 wt % C₄-C₁₂ aliphatichydrocarbons; and about 5 to about 15 wt % cumidine.
 5. The fuelformulation of claim 1 comprising a MON of at least about 100 and an RVPof 38 to 49 kPa at 37.8° C.
 6. The piston engine fuel formulation ofclaim 1 comprising: about 80 to about 99 wt % C₄-C₁₀ alkylates; about 1to about 20 wt % cumidine; and less than about 5 wt % C₆-C₁₂ aromatichydrocarbons.
 7. The piston engine fuel formulation of claim 1comprising: about 80 to about 99 wt % C₄-C₁₀ alkanes; about 1 to about20 wt % cumidine; and less than about 5 wt % C₆-C₁₂ aromatichydrocarbons.
 8. The fuel formulation of claim 7 comprising: about 80 toabout 99 wt % isobutane, isopentane and/or isooctane and about 1 toabout 20 wt % cumidine.
 9. The fuel formulation of claim 8 in which saidcumidine is 4-isopropylaniline.
 10. The fuel formulation of claim 9comprising about 85 to about 95 wt % of isobutane, isopentane and/orisooctane and about 5 to about 15 wt % 4-isopropylaniline.
 11. The fuelformulation of claim 10 being substantially free of C₆-C₁₂ aromatichydrocarbons.
 12. The fuel formulation of claim 10 consistingessentially of about 85 to about 95 wt % isobutane, isopentane and/orisooctane and about 5 to about 15 wt % 4-isopropylaniline.
 13. The fuelformulation of claim 12 comprising a MON of at least about 100 and anRVP of 38 to 49 kPa at 37.8° C.
 14. The fuel formulation of claim 13comprising about 2 wt % isobutane, about 10 wt % isopentane, about 73 wt% isooctane, and about 15 wt % 4-isopropylaniline, and having a MON ofabout 103.5.
 15. The fuel formulation of claim 14 consisting essentiallyof about 2 wt % isobutane, about 10 wt % isopentane, about 73 wt %isooctane, and about 15 wt % 4-isopropylaniline, and having a MON ofabout 103.5.
 16. The fuel formulation of claim 13 comprising about 2 wt% isobutane, about 10 wt % isopentane, about 76 wt % isooctane, andabout 12 wt % 4-isopropylaniline, and having a MON of about
 102. 17. Thefuel formulation of claim 14 consisting essentially of about 2 wt %isobutane, about 10 wt % isopentane, about 76 wt % isooctane, and about12 wt % 4-isopropylaniline, and having a MON of about 102.